1. Field of the Invention
This invention relates to a method of controlling hydrogen concentration in a fluid and more specifically, to a method of controlling hydrogen concentration in the reactor coolant fluid of a nuclear reactor system.
2. Description of the Prior Art
It is well known that certain metals of Group VII of the periodic table are permeable to hydrogen and substantially impermeable to other gases which may be mixed therewith in the commonly found gas mixtures containing hydrogen. The term "hydrogen permeable membrane", as it is used throughout this specification, is intended to include all membranes which are permeable to hydrogen gas and substantially impermeable to other gases. It is also well-known in the art to separate hydrogen from a mixture of such gases by diffusing the hydrogen component of the gas mixture through a hydrogen permeable membrane. Typical of such membranes are those made from palladium alloyed with silver. The mechanism relied upon to achieve this separation is the fact that diffusion of hydrogen across the membrane is sensitive only to a gradient of hydrogen partial pressure across the membrane. Such technology has in the past, been limited to the purification or separation of hydrogen from gas mixtures.
In a typical pressurized water-cooled nuclear reactor, quantities of oxygen are produced during normal operation by the radiolosis of reactor coolant due to intense neutron activity in the core region. Due to the corrosion inducing properties of oxygen, it is necessary that this oxygen be removed from the reactor coolant. In early nuclear plants it was recognized that if a sufficient quantity of hydrogen were introduced into the coolant, it would recombine with the oxygen in the presence of the intense gamma flux in the reactor core.
The most common method of maintaining and controlling the hydrogen concentration in the reactor coolant has been to bleed a portion of the reactor coolant from the reactor coolant system and to pass this coolant through a suitable container in which a hydrogen overpressure is maintained. The pressure of the hydrogen in this tank is dictated by the desired coolant hydrogen concentration.
In typical prior art systems, this tank has been a part of a reactor coolant processing system commonly known as the chemical and volume control system. Such a system removes a stream of coolant from the reactor coolant system and treats it to remove dissolved gases and other impurities. Makeup water and other chemicals are added in the system before the coolant is re-introduced, as required, to the primary reactor coolant system. The coolant treated in the chemical and volume control system is stored in a tank known as the volume control tank prior to being directed back to the reactor coolant system. It is this tank in which a hydrogen overpressure is maintained in order to introduce hydrogen gas into the coolant.
Another of the functions of chemical and volume control systems is to remove any radioactive fission gases such as xenon and krypton which may be present in a dissolved state in the reactor coolant bleed stream. Various techniques of removing these dissolved gases from the coolant have been used in the prior art. In all such systems, however, it has not been possible to selectively remove the radioactive fission gases from the coolant while allowing the hydrogen gas, whose presence is not objected to, to remain dissolved in the treated coolant. As a result, the hydrogen present in the coolant has been removed along with the radioactive fission gases in the gas removal step.
Various methods have been developed and put into use to accomplish the removal of the radioactive fission product gases from the reactor coolant. One method involves the separation, collection and storage of the gases dissolved in the coolant for a period of time in gas decay tanks to permit the decay of the shorter half life radionuclides. These gases are then periodically discharged to the atmosphere or transferred to long term storage tanks. The disadvantages of this system are that: (1) valuable carrier gases such as nitrogen and large volumes of hydrogen must be stored (requiring a large storage capacity) and/or wastefully discharged to the atmosphere along with the less desirable radionuclides; and (2) certain fission products, particularly Kr.sup.85, have long half lives and their discharge to the atmosphere is not desirable even though the amount of such nuclides is small and far below the maximum permissible concentration.
The adsorption of noble gases on charcoal or molecular sieves at ambient temperatures is the process that has been most extensively proposed and used. This is also a method for delaying the release of the noble gases to the atmosphere in order to allow the short-lived isotopes (primarily xenon) to decay. However, it has certain disadvantages such as (1) large beds of charcoal are required; (2) the charcoal burns readily, and molecular sieves are subject to explosion as the result of local heating of adsorbed gases by radioactive particles; and (3) the Kr.sup.85 is released to the atmosphere instead of being concentrated for permanent storage. Additional disadvantages with adsorption on charcoal and molecular sieves are the costs incurred in cooling the bed and the explosion hazards associated with the adsorption of the ozone that is produced by the irradiation of oxygen. Materials that would freeze or condense must be completely removed from the gas prior to its injection into the bed in order to prevent plugging of the equipment.
In cryogenic separation, a third process that has been proposed for use, the noble gases and part of the air, or other carrier gas, are first liquified. Then the noble gases are separated from the bulk gases and are concentrated by fractional distillation. As in all of the low temperature operations, water and other gases that would form solids must be essentially removed prior to the treatment of the noble gases. Solids in the system cause physical difficulties, and the presence of liquid ozone, which is formed from the radiolysis of oxygen, creates an explosion hazard.
A fourth very recently developed process involves separating and collecting the radioactive off-gases and either continuously or periodically placing these gases into contact with a thin palladium-silver membrane across which a hydrogen partial gas pressure is maintained. The hydrogen in the off-gas is selectively diffused across the membrane where it is collected and either discharged, stored for later reuse or reused immediately. On the upstream side of the membrane, the undesirable radionuclides are collected and from there deposited in storage or shipping containers for off-site disposal.
It is significant that each of the above recited prior art processes for removal of undesirable radionuclides treats the entire volume of off-gases which has been removed from the reactor coolant letdown flow by a prior gas stripping step. Since the off-gases withdrawn from the water coolant of a nuclear reactor consist primarily of a large volume of hydrogen . . . and a very small volume of the noble gases produced as fission products within the reactor core and leaked into the water coolant, each of the above-described separation processes requires gas stripping and gas separation apparatus of sufficient capacity to handle all of the dissolved gases present in the reactor coolant letdown flow.
Because all of the residual hydrogen gas remaining in the reactor coolant letdown flow is removed from the coolant during the gas removal step, the burden on the apparatus and process for introducing hydrogen into the coolant is accordingly increased, i.e. it must bring the level of hydrogen concentration up to the desired level from a point of zero hydrogen concentration. Generally, the removal of dissolved gases has been carried out on an intermittent basis and thus the hydrogen overpressure technique described previously has been adequate to maintain the desired hydrogen level in the primary coolant loop. Currently, however, reactor systems are being designed on the basis of continuous gas stripping and a more severe requirement for rehydrogenation, which current techniques cannot meet, have been imposed. It has also been postulated that hydrogen introduced into the flow upstream of the pumps which re-introduce the treated coolant to the primary coolant loop, may be outgas at the pump suction and cause performance deterioration. These pumps are commonly referred to as the charging pumps.
As a result of these concerns it has been suggested that hydrogen be introduced into the returning coolant flow downstream of the charging pumps in a region of high pressure. The metering of the relatively small hydrogen flow required at the high pressures present here would be understandably difficult. As an example, a typical volume flow at such a point in the system would be 0.191 SCFM, into a region at 2250 psig. At this pressure, this would be an actual volume flow of only 0.00127 cubic feet per minute. There would also be the obvious requirement to store and supply hydrogen at high pressures; a hazardous and expensive requirement.